Long before the San Francisco Bay Regional Water Quality Control Board (RWQCB) implemented new permit requirements last fall, mandating that agencies discharging into the Bay collectively reduce nutrient discharges by 40% from 2022 levels within the next decade, the East Bay Municipal Utility District (EBMUD) initiated research and planning to evaluate the most cost-effective methods for reducing nutrients through both sidestream and mainstream nutrient removal techniques.
One of these projects is to assess its capacity to remove total inorganic nitrogen (TIN) by utilizing the existing high-purity oxygen-activated sludge (HPOAS) system. Using HPOAS for nitrogen removal involves several complexities. After years of experimentation, EBMUD is implementing a biological nitrogen removal (BNR) process that is demonstrating great success in effectively reducing nitrogen in wastewater discharges.
EBMUD adjusted operations and utilized its existing infrastructure to encourage the growth of bacteria that break down inorganic nitrogen compounds, including ammonia, nitrite, and nitrate, which are common in wastewater.
“Every year since we initiated the program, we’ve increased the plant flow through the BNR process. In 2024, we achieved 75% of our plant flow,” said Justin Shih, senior engineer for EBMUD, “and we recorded our lowest total inorganic nitrogen discharge since monitoring began in 2012.” This achievement reflects the collaborative and persistent efforts across EBMUD’s Wastewater Department, including Operations, Maintenance, Engineering, and Laboratory staff.
EBMUD provides wastewater treatment services to approximately 740,000 people within an 88-square-mile area of Alameda and Contra Costa counties along the eastern shore of the Bay, stretching from Richmond in the north to Oakland. Additionally, EBMUD’s water system serves around 1.4 million people across a 332-square-mile area, which includes the major cities of Oakland and Berkeley.
The District’s Main Wastewater Treatment Plant (MWWTP) treats approximately 60 MGD of wastewater each day. Since the 1970s, it has utilized HPOAS for secondary treatment. The system comprises of eight reactor trains (each with four stages) and twelve secondary clarifiers, which have a rated capacity of 168 MGD during wet weather. High-purity oxygen is produced from an on-site cryogenic air separation plant. In the early 2000s, the first stage of each reactor was converted into an anaerobic selector to manage sludge bulking.
MWWTP typically operates with a mean cell residence time (MCRT) of 1 to 1.5 days; however, much longer MCRTs are necessary to support nitrifying bacteria—the primary microorganisms that oxidize inorganic nitrogen. The resulting increase in solids inventory can create challenges for the existing infrastructure, including a higher solids load on the clarifiers.
Additional concerns include insufficient alkalinity for complete nitrification—oxidation of ammonia to nitrite and then to nitrate—lack of internal recycling for denitrification, which involves the reduction of nitrate and nitrite to nitrogen gas, potential for rising sludge blankets in the clarifiers due to gas formed from denitrification, nitrite effects on hypochlorite disinfection, and managing process start-up each year.
“EBMUD has performed full-scale BNR testing during dry weather for the past several years using its existing HPOAS process, and in 2024 began testing during wet weather” Shih said.
The current HPOAS reactor and clarifier setup, using a series of valves, gates, and pumps, can be physically divided into two parallel processes with entirely independent biomass. This design allows for comprehensive testing of higher MCRTs while maintaining the existing process for standard treatment.
“There is the ability to configure a 25/75, 50/50, or 75/25 split. Both sides’ effluent combines before disinfection, dechlorination, and discharge,” he said.
This testing effort will be summarized in a publication, expected in 2025, that documents the startup and operation of EBMUD’s BNR process. While becoming increasingly rare, many wastewater agencies continue to successfully operate and maintain their HPOAS systems.
Shih believes that, along with the ongoing BNR process optimization efforts, EBMUD will likely require additional capital improvements to ensure a robust and reliable treatment process, and to account for future growth in EBMUD’s service area or changes to permit requirements. The potential capital improvements will be evaluated through an update of EBMUD’s Nutrients Master Plan. These efforts will solidify our approach to meeting the final effluent limitations starting October 2034.
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